A little bit of work but for fungus gnats I prefer to do a complete repot. Clean the pot, and get new soil. While the plant is uprooted, I give it a little bath in the sink with peroxide, a plant safe soap, and water.

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I recommend just getting some gnat traps, mine are cute little pieces of paper I just stick into the soil and they catch all of the gnats

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Should be fine, make sure it’s 3% or less

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You can try putting a piece of a mosquito dunk in your watering can for a bit and then water the plant with that

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